:: Synchronizing package databases... core is up to date extra is up to date community is up to date multilib is up to date:: Starting full system upgrade...resolving dependencies...looking for inter-conflicts...

Re: error: key "E6B456CAF15447D5" could not be looked up remotely

I belive that you will be able to upgrade all except the texmaker package. Hope that helps some what...

I have no clue of how to find out what package has that "broken" signature If you have like 80 packages that needs to be upgraded... I had to filter out each package one by one with # pacman --ignore to find it, real pain in the a**.